As the minimum pitch required to be resolved has decreased more aggressive off-axis illuminations have been required. These aggressive illuminations allow the minimum pitch to be resolved but have also decreased the depth of focus of larger pitches (typically pitches 1.5 times the minimum pitch). One of the methods employed to improve printability and image fidelity of features with pitches greater than minimum is use of printed resolution assist feature (PRAF), which has dimensions greater or equal to that of the minimum feature size. The difference between a PRAF and a sub-resolution assist feature (SRAF) is that the dimensions of the PRAF are large enough that they are resolvable, while the SRAF are not resolvable but do enhance the aerial image of the desired features. As a consequence, the PRAF is printed on a photoresist, while the SRAF is not printed on a photoresist despite the fact that both are present on a lithographic mask.
Double exposure lithography, in which a first photoresist is applied and patterned followed by application and patterning of a second photoresist, provides a method of patterning a complex pattern with enhanced resolution. Meiring et al., “ACLV driven double-patterning decomposition with extensively added printing assist features (PrAFs),” Proc. SPIE Int. Soc. Opt. Eng. 6520, 65201U (2007) discloses a double exposure scheme, in which PRAFs are employed in the first exposure and then removed in the second exposure. This can be very important in modern circuit designs where decreasing the area that any give function takes up can allow for more functionality to be placed within a given chips area. These PRAFs to date have been lines. In other words, extra lines are added to a design pattern so that the resulting pattern is more like a grating of lines. Because the pattern is more regular in nature the aerial image has more contrast than the pattern where PRAFs have not been added. Because these added lines are not desirable in the final pattern on wafer a second exposure is done which removes the PRAFs.
This combination of double exposure and PRAFs has allowed for even more aggressive illumination to be used (conventional to annular to quasar to custom diffractive optical elements) which has resulted in tighter minimum pitches being resolved. However, many solutions that have been developed for enhancing the process window of layers such and contract holes or metal lines (which require resist holes and/or trenches) with clear PRAFs have drawbacks, Dah-Chung Owe-Yang et al., “Double exposure for the contact layer of the 65-nm node”, Proc. SPIE Int. Soc. Opt, Eng. 5753, 171 (2005) discloses alcohol based photoresist to be used for the second exposure to prevent intermixing of the second photoresist with the first photoresist remaining after the first exposure. These alcohol based photoresist are highly undesirable in a manufacturing facility. Alternatively, extra features may be physically present on the wafer, which is present only for supporting the printing of the design. This can usually not be done since the extra features interfere with the functionality of the chip.
In other words, the known methods described above for adding holes or trenches to a pattern to make the pattern more grating like and enhance the aerial image of the design features either are not desirable from a manufacturing perspective or do not eliminate the cPRAF. U.S. patent application Ser. No. 11/733,412 describes a manufacturable method that employs a trilayer photoresist system or a bilayer photoresist system for the first and second exposures, respectively. The bilayer photoresist fills the holes or trenches of the unwanted features printed in the first exposure. Therefore, a conventional organic-based single layer photoresist or an inorganic hardmask may not be employed according to this method.
In view of the above, there exists a need for a method of forming a lithographic pattern containing cPRAF features while employing conventional organic-based single layer resists.
Further, there exists a need for a method of forming a lithographic pattern containing cPRAF features while employing a common hardmasks used in the semiconductor industry.